A color facsimile communication scheme is recommended by ITU-T, and color facsimile apparatuses have been put into practical use.
Color facsimile communication uses image data compressed by a JPEG (Joint Photographic Experts Group) scheme. As the color space of this image data, a uniform color space which is independent of image input/output apparatuses such as scanners and printers is used, so that the color information of an image to be transmitted is not degraded by the color reproducibility of such apparatuses. International standard color facsimile communication requires the use of a uniform color space Lab defined by CIE (Commission Internationale de l'Eclairage).
Color facsimile transmission procedures are disclosed in various materials. For example, Japanese Patent Laid-Open No. 11-122496 explains as follows.
An image is read, and the image data obtained by A/D conversion is subjected to shading correction. A color correction unit reads out color correction data corresponding to the reading characteristics of an image reading device from a memory, and performs color correction. A color converter converts the color space of the color-corrected image data from an RGB color space into a CIE Lab color space, and the converted image data is subjected to JPEG compression. After that, the JPEG-compressed data is transmitted.
In color facsimile communication as described above, a read image is converted into a CIE Lab color space.
With the spread of the Internet, it is becoming more and more necessary to transmit images by attaching them to e-mail. Japanese Patent Laid-Open No. 9-325924 has proposed a technique which transmits an image read by a facsimile apparatus by converting the image into a TIFF (Tagged-Image File Format) and attaching the converted image to e-mail. An outline of this technique will be described below.
A user sets the originals of images to be transmitted on the transmission table of a facsimile apparatus and enters a password for checking whether the user is a registered user. The user then selects the transmission destination of the images from a communication partner table, inputs the abbreviated number of the destination, and presses a start key, or presses a one-touch key with which the communication partner is registered.
The facsimile apparatus reads the images of the originals on the original table one by one, encodes the read images into G3 image data by an MMR (Modified Modified READ) scheme, and stores the image data of all pages into an image memory. The facsimile apparatus reads out the designated transmission destination from the communication partner table and transmits the image data along with e-mail.
To transmit the images by attaching them to e-mail, the G3 image data is converted into TIFF image data. That is, TIFF class F header information is added to the head of the G3 image data to form TIFF image data. Since this TIFF image data is binary data, the data is further converted into text data.
When the transmission data in an e-mail form is completed, the facsimile apparatus connects to a provider by dial-up. When the line to the provider is set, the facsimile apparatus logs in by a PAP (Password Authentication Protocol), and transmits the e-mail by an SMTP (Simple Mail Transfer Protocol). After the transmission is completed, the facsimile apparatus logs off and disconnects the line.
As described above, image data used in color facsimile communication is obtained by JPEG-compressing image data having a CIE Lab color space. However, image data used in the Internet is obtained by JPEG-compressing image data having a YCbCr color space. Accordingly, when image data used in color facsimile communication is directly attached to e-mail and transmitted, a computer which has received the image data cannot reconstruct a correct image.
Japanese Patent Laid-Open No. 11-27543 has disclosed a color space switching means for use in a color facsimile apparatus. Japanese Patent Laid-Open No. 11-27543 has also disclosed a method of switching two color conversion processes: a color conversion process which converts, with little error, image data required to have high image quality, and a high-speed color conversion process for image data required to have only poor image quality. However, these means and method do not switch different color spaces in accordance with the communication methods as described above.